In recent years, the performance of silicon-based semiconductor devices has steadily increased as the design rule has become smaller. As a result, heat dissipation from individual transistors and metal lines connecting between transistors has become a problem. Solutions that have emerged to address this problem include thinning the back side of the silicon substrate to a thickness of about one hundred to several hundred microns following device fabrication and mounting a giant fan over the chip to promote heat dissipation or running water-cooling tubes around the device.
However, even when the silicon is thinned, the region where devices are fabricated only extends to a depth of about several microns from the surface and regions other than this act as heat reservoirs; hence, the efficiency from the standpoint of heat dissipation actually worsens. Lately, silicon-on-insulator (SOI) wafers and the like used in devices such as high-performance processors have a structure wherein an insulating layer of SiO2 is situated immediately below the device active layer. But SiO2 has a low thermal conductivity of 1.38 W/m·k, making this approach highly problematic from the standpoint of heat dissipation. In addition, silicon substrates, owing to their dielectric properties, undergo a large power loss in the high-frequency region, and so there are limitations to their use.
Silicon on sapphire (SOS), which uses a sapphire substrate, is noteworthy because it has a good heat conductivity and a small power loss in the high-frequency range. However, one drawback is that, because the sapphire substrate is transparent in the visible light region, it does not respond to optical sensors used for verifying the presence or absence of the substrate and for wafer positioning in device fabrication processes. Another problem is the high cost of the sapphire substrate. In addition, the difference in coefficient of thermal expansion relative to silicon is large, and so warpage tends to arise during heat treatment of the composite substrate and film formation, making it difficult to achieve larger wafer diameters.
Examples of inexpensive substrates which are opaque to visible light and have good heat conductivity include ceramic sintered bodies made of silicon nitride, aluminum nitride or the like. However, because these substrates are obtained by cementing a silicon nitride or aluminum nitride powder with a sintering aid, metallic impurities such as iron and aluminum included in the powder, or the sintering aid itself (e.g., alumina), become causes of contamination in the device fabrication processes, making the use of such substrates difficult.
Prior technical literature relating to the invention includes WO 2013/094665 (Patent Document 1).